1. Field of the Invention
Apparatuses and methods relating generally to printed circuit board testing, and more particularly to testing dense, high speed, printed circuit boards.
2. Description of Related Art
During the manufacture of printed circuit board assemblies, testing is performed at one or more stages to ensure that the finished product functions adequately. In some manufacturing operations, bare printed circuit boards are first tested before any components are mounted to them. Once components are attached, the populated printed circuit boards are tested again, either simply to ensure that the components are properly connected to the printed circuit boards or to test the functionality of the assembly.
Testing may be performed by connecting test probes to signal transmission paths on the board. These test pads provide electrical test access points for test equipment. In operation, the printed circuit board under test is placed in a fixture that has test probes aligned with the test access points. These probes can sometimes be mounted in fixed locations in the fixture such that each probe makes contacts with a single test pad during an entire test. In this case, multiplexing circuitry within the test system couples test signals to or from different test points at different times during the testing process. Though, some test systems have included robotic arms that moved probes to align with different test pads during testing.
Regardless of whether fixed or moving probes are used, these probes should make contact with enough locations on the printed circuit board so that all, or a very high percentage of the printed circuit board, can be tested. Some manufacturing methods lead to printed circuit assemblies that have many locations where connection for a test system can be made. For example, “through hole” component mounting results in exposed component leads on at least one side of the printed circuit boards that provides a large number of points that can be easily probed for test access. Though, because of the size of printed circuit boards using through hole mounting and other drawbacks, this approach is not widely used. Rather, surface mount techniques are more commonly used.
In surface mount techniques, components are first placed on the printed circuit board with conductive leads (sometimes called “pins”) of the components aligned with pads on the surface of the printed circuit board. Solder, or solder paste that turns into solder when heated, is placed between the lead and pad and then melted in a reflow operation. When the solder cools, it forms a connection between the lead and the pad. Surface mount leads can be much smaller than through hole leads, making it more difficult to probe surface mount leads for testing. In addition, some surface mount techniques, such as Ball Grid Array attachment, have leads on the bottom surfaces of components, which cannot be contacted during a test. Thus, changes in manufacturing processes have reduced test access.
One approach to testing that reduces the need for test access points is called capacitive testing. In a capacitive “opens” test, a probe that includes a sense plate is pressed against a component with a pin to be tested. A test signal is generated at a point on the circuit assembly that should be connected to the pin under test in a properly manufactured circuit assembly. If the pin under test is properly connected to the circuit assembly, the test signal will propagate from the circuit assembly through the pin to conducting structures within the component. Though the probe plate is separated from those conducting structures, the test signal can capacitively couple to the probe plate. During a test, the signal received at the probe plate is analyzed to determine whether the test signal has been capacitively coupled to the probe plate, indicating that there is a good connection between the assembly and the pin of the component.
Capacitive testing is useful for testing for shorted or opens pins, but does not generally provide test access for other types of testing. Also, not all points at which test signal is to be injected or measured during a test can be accessed with capacitive testing. Thus, there is still a need for test access points.
Thus, in many circuit assemblies, test access points were incorporated into the surface of the printed circuit board. Test access points in the past were generally circular targets, approximately 28 mils to 35 mils in diameter, connected to traces on the printed circuit board. In some cases, these targets could be via pads surrounding vias already provided in the printed circuit. Though, in many cases, these targets were added test pads.
These test pads add capacitance to the signal traces to which they are connected. The added capacitance changes the impedance of the signal traces. With advances in integrated circuit technology, signal speeds are becoming greater. For high speed signals, changes in impedance can cause reflections or other undesirable effects that interfere with reliable signal transmission. As a result, test pads may limit the signaling speed that may be reliably achieved on a transmission path and therefore disrupt operation of a printed circuit board that has been modified to include test pads.
In addition, as manufacturing technologies allow smaller printed circuit boards, test targets are increasingly difficult to contact reliably and repeatably, especially when a test fixture may contain several thousand such probes. An alternative to probing test pads is to place solder bumps or beads on a surface of a printed circuit board. The solder bumps extend above the surface of the board such that a flat probe can contact a solder bead to couple signals to or from the printed circuit board for testing. The flat probe can be wide to companionate for any imprecision in the placement of the solder bump or the pad. Consequently, the solder beads can be made smaller than test pads conventionally used for test access.